When the pickup head of an optical information recording/reading apparatus writes and reads pits of a compact disk, an automatic power control loop is used to control and adjust the laser power of the laser diode within the pickup head. A pickup head comprises a laser diode, a laser diode driver and a laser power detector. The laser diode provides a laser source to read or write data on the compact disk. The laser power detector detects the laser power of the laser beam reflecting from the compact disk to output a corresponding laser power detection signal.
The pickup head uses different laser powers when it is writing or reading. The power in writing is higher than the power in reading. The laser powers are also different depending on writing/reading a land storage track or a groove storage track. It is important to control the power of the laser emitted by the laser diode. The laser diode is driven by the laser diode driver. Hence, controlling the laser diode driver can further control the laser power.
The automatic power control loop controls the laser diode driver to output a electric current to drive the laser diode for emitting a laser bean with an adequate laser power.
The laser power detection signal outputted by the laser power detector and the laser power of the detected laser beam are in inverse proportion. The laser power detection signal will be sampled and held by a sample/hold circuit of the automatic power control loop, and then transmitted to a pre-amplifier for amplifying to generate a pre-output signal. The pre-output signal is subtracted by a predetermined reference signal in an adder. If the result value does not equal to zero, the laser power of the laser beam still needs to be adjusted. A post-amplifier can adjust the pre-output signal and generate a power control signal. The post-amplifier transmits the power control signal to the laser diode driver for emitting an adequate laser power. In an ideal condition, when the laser diode doesn't emit the laser beam, the pre-output signal is the predetermined reference signal.
In general, the prior art control and calibration method for the laser power emitted by the laser diode can adjust the laser power very well. However, in the prior art, when the laser power detector generates a laser power detection signal with an offset, the automatic power control loop can't calibrate immediately. Through the amplifier, the offset is amplified and may be beyond the range that the adder can adjust. Therefore, a wrong power control signal is produced resulting in a wrong laser power wrong. Consequently, mistakes occur in writing/reading data of the compact.